The measurement of currents using light waves in an optical fiber wound around a conductor has e.g. been described in EP 856 737.
In this device, two orthogonal, linearly polarized light waves are sent through an electro-optic phase modulator for introducing an ac phase modulation and then over a polarization maintaining fiber (PMF) to a site of measurement, where they are converted by a retarder to two circularly polarized light waves of opposite orientation. These circularly polarized light waves pass through a measuring fiber wound around a conductor. At the end of the measuring fiber, a reflector sends the light waves back to the retarder, where they are converted back to two linearly polarized light waves. The returning light from the retarder is separated from the original light in a beam splitter and sent to a detector.
Alternatively, devices based on fiber gyro modules can be used.